When thermosetting resins are cured by a controlled process such as heat, it is often necessary to monitor the degree to which they are cured, so as to know whether to continue the curing process or terminate it. If the resin is undercured, the resulting article will be weak and have poor properties, and if the resin is overcured, time and energy will be wasted and the properties of the resulting article may again begin to deteriorate. One way to monitor the degree of cure is by means of dielectric techniques, where the dielectric constant and dissipation factor of the resin is monitored as it is being cured. However, this method requires that metal electrodes intrude into the resin or contact the resin. Also, due to a poor signal to noise ratio, it is not an ideal procedure for use with graphite-epoxy composites.
The curing process can also be monitored by means of ultrasonic waves. An ultrasonic transmitter is placed on one side of the container and an ultrasonic receiver is placed on the other side and ultrasonic waves are passed through the resin as it cures. While this technique eliminates the intrusion of an electrode into the resin, because of diffusion of the ultrasound beam it is also not extremely sensitive to the physical and chemical changes that are occurring as the resin cures.